loaders: PNG: Add support for 16 bit Grayscale.
loaders: PNG: Add support for 16 bit Grayscale.

Since we have PIXEL_G16 in config, we can make use of it.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim